A glass is defined as a non-crystalline solid exhibiting the glass transition phenomenon. It has an amorphous structure due to liquid solidification during synthesis and only has a short-range order. A glass has a low resistance to mechanical shocks due to the presence of defects therein, it is also defined as brittle and fragile. A distinction is made between a plurality of glass families according to the chemical composition thereof: oxide, fluoride, metallic, organic glasses and chalcogenide glasses. These latter are frequently based on sulphur (S), selenium (Se) or tellurium (Te).
Chalcogenide glasses are currently studied for their wide transmission range extending from the visible to the infrared range. They are particularly marketed for thermal imaging. Chalgogenide glasses have relatively weak chemical bonds, giving said glasses poor thermomechanical properties. In order to offset these properties, glass-ceramics have been manufactured by means of a mere heat treatment of chalcogenide glasses. Now considered to be material in their own right, glass-ceramics are defined as composites consisting of crystals within a vitreous matrix. Their mechanical (toughness, hardness, etc.), optical and thermal properties are modified by the creation of crystallites of varied sizes within the structure.
The document WO 2005/005334 describes chalcogenide glass-ceramics exhibiting transparency in the infrared range.
The photoelectric effect consists of creating electron/hole pairs in a material when said material is exposed to illumination. The electrons are excited by a photon flux, due to the illumination, and are then more or less free to move in the structure. The energy of the absorbed photon creates a pair of free carriers: an electron (or photoelectron) in the conduction band and a hole in the valence band. The electron/hole pair then generates an electric current called a photocurrent. The potential applications of the photoelectric effect are numerous. The photocurrent may either be measured for use as detectors (photodiode, photoelectric cell) or collected to supply electricity (photovoltaic cell).
In respect of photovoltaic applications, the material used most at the present time is doped silicon (p and n) which is found in almost 90% of solar panels produced worldwide. However, this material has a relatively low light energy conversion efficiency in relation to the production cost thereof.
In respect of photocatalytic applications, titanium dioxide (TiO2) is currently the most studied photocatalyst. However, this material has a relatively low sunlight absorption rate (approximately 4%) since it has a band gap corresponding to light irradiation in the near UV spectral region.
The document FR-B1-2 831 467 describes a photocatalyst comprising TiO2.
The aim of the invention is particularly to provide a simple, effective and economical solution for the abovementioned problems of the prior art by means of a material suitable for use for photovoltaic and photocatalytic applications and not exhibiting the abovementioned problems of the prior art.
For this purpose, the invention relates to a chalcogenide glass-ceramic whose composition comprises, as a molar %,
Ge + Sn + Pb3-25Sb + In + As + Bi10-35 Se + Te40-65 M2-17X2-17wherein M is a transition metal such as Cu and X is a halogen such as I, Cl or Br, and the sum of all the molar percentages of the composition is equal to 100,the glass-ceramic comprising at least one crystalline phase, characterised in that the crystallisation rate and the dimensions of the crystals in the crystalline phase are such that the crystals are substantially in contact with each other in such a way that this crystalline phase has an electrical conductivity greater than or equal to 10−4 s·cm−1 which increases under lighting due to the creation of charge carriers within the crystalline phase.
The crystalline phase thus has semiconductor properties, the charge carriers created within the crystalline phase under lighting being movable and inducing the formation of a photocurrent in the glass-ceramic. The lifetime of the charge carriers found in minority is for example typically between 10-20 μs.
The invention thus relates to a material having particularly advantageous photoelectric properties, in particular since it is suitable for generating a relatively strong photocurrent when the material is subjected to light radiation, for example visible. This material does not exhibit the drawbacks of the above-mentioned materials of the prior art in particular as it can absorb wavelengths in the UV and visible range and the production method thereof is relatively simple and inexpensive.
The inventors observed that the electrical properties of a chalcogenide glass may be markedly enhanced due to the formation of crystallites in which the charge carriers are movable. The size of the crystals and the crystallisation rate may be optimised so that the crystals are substantially in contact with each other. This way, the inventors observed a continuous increase in the electrical conductivity of the glass-ceramic as a function of ceramisation time (FIG. 2). In this case, the chalcogenide glass-ceramic is capable of generating movable charge carriers when exposed to light, creating a photocurrent. The crystals generated thus fundamentally change the electrical properties of the initial glasses which are electrical insulators.
In the present application, the term charge carriers denotes a particle carrying an electrical charge (electrons carry a negative charge and holes carry a positive charge). When they move, charge carriers create an electrical charge. In an n-type semiconductor material, the charge carriers found in majority are electrons and the charge carriers found in minority are holes.
The term crystals or crystallites substantially in contact denotes crystals which are in physical contact with each other or which are in the immediate vicinity of each other (the inter-crystal distances are then very small, for example of the order of one nanometer).
So that the charge carriers generated in the glass-ceramic can move in crystals and from crystals to adjacent crystals, it is preferably necessary to prevent the vitreous phase, which is electrically insulating (due to the fact that the charge carriers created are immobile in this phase), from extending between the crystals and thus impeding the mobility of the charge carriers. Each crystal may not be in contact with all the other crystals and all the crystals are not necessarily in contact with at least one further crystal. The glass-ceramic may comprise one or a plurality of crystalline phases, each crystalline phase may comprise a certain quantity of crystals which are in contact with each other and are suitable for generating movable charge carriers and thus a photocurrent. The greater the number of crystals in contact, the higher is the conductivity of the glass-ceramic.
Although chalcogenide glasses are electrically insulating, the chalcogenide glass-ceramics according to the invention thus exhibit a significant electrical conductivity which increase under illumination. This is enabled by a high absorption of visible light by the material, which produces charge carriers susceptible of moving within the crystalline phase. The charge carriers have a long lifetime (typically between 10-20 μs) and do not recombine rapidly, giving the material an increased electrical conductivity when illuminated. The electrical conductivity of the glass-ceramics according to the invention may be between 1 and 10−4 s·cm−1, preferably between 10−4 and 10−1 s·cm−1, more preferentially between 10−3 and 10−1 s·cm−1, and for example between 10−2 and 10−1 s·cm−1.
Controlled crystallisation of chalcogenide glasses thus makes it possible to increase the electrical conductivity thereof and the photoelectric properties thereof significantly. The glass-ceramics according to the invention are thus comparable to semiconductors. Moreover, as explained in detail hereinafter, the glass-ceramic may have n type, p type, or p and n type behaviour.
The same glass-ceramic may comprise a plurality of separate crystalline phases, which may have the same behaviour (p/n) or mutually different behaviours.
The applications of this type of material relate generally to the conversion of solar energy into electrical or chemical energy.
The conversion of solar energy into electrical energy represents the photovoltaic effect. Chalcogenide glass-ceramics have the advantage of having a direct band gap (between 1 and 2 eV approximately) and thus relatively high light absorption coefficients. Moreover, they may be processed in the form of a thin layer (thickness in the region of 1 to 2 μm for example), which reduces the amount of material used.
Moreover, the charge carriers generated within the glass-ceramic may be p type, n type, or p and n type. The chalcogenide glass-ceramics according to the invention may thus be equipped with p-n junctions, particularly useful for producing photovoltaic solar cells for example.
The conversion of solar energy into chemical energy represents the photocatalytic effect. The chalcogenide glass-ceramics according to the invention have the advantage of having a high chemical stability.
The chalcogenic glass-ceramic according to the invention may have one of the following compositions: GeSe2—Sb2Se3—CuI; SnSe2—Sb2Se3—CuI; PbSe2—Sb2Se3—CuI; GeSe2—As2Se3—CuI; GeSe2In2Se3—CuI; GeSe2—Bi2Se3—CuI; GeTe2—Sb2Se3—CuI; GeSe2—Sb2Se3—CuCl and GeSe2—Sb2Se3—CuBr.
The composition of the glass-ceramic according to the invention comprises for example, as a molar %,
Ge3-25Sb10-35 Se40-65 Cu2-17I2-17the sum of all the molar percentages of the composition being equal to 100.
The glass-ceramic may be free from one or a plurality of the following elements: Ga, Ge, S, Cs, Zn, Cd, Rb, Na, K, B and La. Some of these elements may not be used due to their toxicity (such as Cd) and others due to the rarity and thus their cost (such as Ge, In, Ga).
In one embodiment of the invention, the glass-ceramic has the following composition GeSe2—Sb2Se3—CuI, wherein the respective molar percentages are:
GeSe230-50, preferably 35-45, and for example 40Sb2Se330-50, preferably 35-45, and for example 40CuI10-30, preferably 15-25, and for example 20
The crystallisation rate (which may be defined as being the ratio of the total volume occupied by the crystals or crystallites over the volume of the crystalline phase of the glass-ceramic) of the crystalline phase may be greater than 50%, preferably greater than 70%, and more preferentially greater than or equal to 80%, in volume. The crystallisation rate may be between 50 and 100%, preferably between 60 and 95%, more preferentially between 70 and 90%, and for example between 80 and 90%.
The crystals of the crystalline phase may have a mean diameter between 0.1 μm and 10 μm, preferably between 0.5 and 5 μm, and more preferentially between 1 and 3 μm. In the case wherein the crystals have an elongated shape (for example needle-shaped), their length may be between 0.1 and 10 μm, and preferably between 1 and 5 μm. The form factor of the crystals may have an influence on their content and their size in the crystalline phase, in order for the crystals to be substantially in contact with each other and to enable the crystalline phase to have the abovementioned photoelectric properties.
The present invention also relates to a method for producing a glass-ceramic, from a chalcogenide glass wherein the composition comprises, as a molar %,
Ge + Sn + Pb3-25Sb + In + As + Bi10-35 Se + Te40-65 M2-17X2-17wherein M is a transition metal, such as Cu, and X is a halogen, such as I, Cl or Br, and the sum of all the molar percentages of the composition is equal to 100, characterised in that it comprises a step of subjecting the glass to a heat treatment the duration, and time whereof are determined to create at least one crystalline phase in the glass, the crystallisation rate and the dimensions of at least some crystals of the crystalline phase being such that the crystals are substantially in contact with each other in such a way that this crystalline phase has an electrical conductivity greater than 10−4 s·cm−1, which increases under lighting due to the creation of charge carriers within the crystalline phase.
The method according to the invention may comprise one or a plurality of the following additional steps:                i) selective chemical etching of the glass-ceramic with a view to increasing the specific surface area thereof; and        ii) grinding of the glass-ceramic, which makes it possible to increase the specific surface area thereof considerably.        
The increase in the specific surface area of a glass-ceramic makes it possible to increase the surface area of the glass-ceramic, which may be exposed to illumination, which enhances the photocatalytic efficiency. This increase in the specific surface area may be produced by selective chemical etching of the glass-ceramic surface making use of a difference in solubility between the crystalline and vitreous phases, thus creating a nanoporous surface.
The duration of the heat treatment may be dependent on the treatment temperature. It is for example between 1 and 15 hours and preferably between 3 and 6 hours. The temperature of the heat treatment is for example greater by more than 10° C., preferably by more than 30° C., and more preferentially by at least 50° C. than the glass transition temperature (Tg) of the glass. The temperature of the heat treatment may be between Tg+10° C. and Tg+150° C., preferably between Tg+20° C. and Tg+100° C., more preferentially between Tg+40° C. and Tg+70° C.
The duration of the heat treatment may be greater than 1 hour, preferably greater than 2 hours, and more preferentially greater than 3 hours so that the crystalline phase can generate a p and n type photocurrent. The duration of the heat treatment may be between 1 and 10 hours, preferably between 2 and 8 hours, and more preferentially between 3 and 6 hours. The invention further relates to the use of a glass-ceramic as described above, for producing electricity by means of the photovoltaic effect, or for decomposing or processing a chemical or biological substance by means of the photocatalytic effect, and in particular decomposing pollutants, hydrogen generation, CO2 reduction.
The invention finally relates to a product chosen from a photovoltaic cell, means for decomposing or processing a chemical or biological substance such as a pollutant, water or CO2, characterised in that it comprises at least one glass-ceramic as described above.
Numerous publications describe in detail the principle of water decomposition for hydrogen production (see for example Semiconductor nanostructure-based photoelectrochemical water splitting: A brief review de Yongjing Lin et al., Chemical Physics Letters 507 (2011) 209-215).